The solar system is densely packed with planets and also contains an asteroid and a Kuiper belts, remnants from the planet-formation epoch. Are planetary systems with high-mass planets any different in terms of remnant planetesimal belts from those with low-mass planets or those with no known planets? What does this tell us in terms of planetary system formation and evolution?

Image credit: Lynette Cook

Planetesimals are the building blocks of planets, and mid and far-infrared observations with Spitzer and Herschel indicate that at least 10–25% of mature stars (10 Myr to 10 Gyr) harbor planetesimal disks with disk sizes of tens to hundreds AU (this frequency is a lower limit because the surveys are limited by sensitivity). The evidence for planetesimals comes from the presence of circumstellar dust: because the lifetime of the dust grains (<1 Myr) is much shorter than the age of the star ( >10 Myr), it is inferred that the dust cannot be primordial but must be the result of steady or stochastic dust production generated by the collision, disruption, and/or sublimation of planetesimals, like the asteroids, comets and Kuiper belt objects in our solar system. The presence of these debris disks in both single- and multiple-star systems, and around A- to M-type stars (also around the progenitors of white dwarfs), spanning several orders of magnitude difference in stellar luminosities, imply that planetesimal formation, a critical step in planet formation, is a robust process that can take place under a wide range of conditions. It is therefore not surprising that in some cases planets and debris disks coexist. But are dust-producing planetesimal disks more or less common around stars with planets? Using the evolution of the solar system as a model, in its early history, a star with planetary companions could be expected to be surrounded by a massive debris disk produced by the planetesimal swarm that formed the planets, the latter exciting planetesimal collisions and dust-production while undergoing orbital migration. On the other hand, at a later stage, the star could harbor a sparse dust disk after the dynamical rearrangement of the planets is complete and the planetesimal swarm has undergone significant dynamical clearing. Do observations support these trends?

Because the study of the planet-debris disk correlation could shed light on the formation and evolution of planetary systems and may help “predict” the presence of planets around stars with certain disk characteristics, we have carried out a statistical study of an unbiased sub-sample of the Herschel DEBRIS and DUNES debris disk surveys, to assess whether the frequency and properties of debris disks around a control sample of solar-type stars are statistically different from those around stars with planets. Out of the 466 and 133 stars in the DEBRIS and DUNES samples, respectively, we have selected a subsample of 204 FGK stars located at distances <20 pc (to maximize survey completeness), with ages >100 Myr (to avoid introducing a bias due to disk evolution), and with no binary companions at <100 AU (to avoid introducing a bias due to the observed differences in both disk frequency and planet frequency between singles and multiples). The debris-disk frequency within this unbiased sample is 0.14 +0.3/-0.2 .

In this clean sample, we don’t find any evidence that debris disks are more common or more dusty around stars harboring high-mass planets (> 30 MEarth) compared to the average population. Overall, this lack of correlation can be understood within the context that the conditions to form debris disks are more easily met than the conditions to form high-mass planets, in which case one would not expect a correlation based on formation conditions; this is also consistent with the studies that show that there is a correlation between stellar metallicity and the presence of massive planets, but there is no correlation between stellar metallicity and the presence of debris disks. Another factor contributing to the lack of a well-defined correlation might be that the dynamical histories likely vary from system to system, and stochastic effects need also to be taken into account, e.g., those produced by dynamical instabilities of multiple-planet systems clearing the outer planetesimal belt or the planetesimal belt itself triggering planet migration and instabilities.

Regarding low-mass planets (< 30 MEarth), one would expect that if the planets formed in the outer region and migrated inward, low-mass planets would have been inefficient at accreting or ejecting planetesimals, leaving them on dynamically stable orbits over longer timescales. On the other hand, high-mass planets would have been more efficient at ejecting planetesimals, leaving behind a depleted population of dust-producing parent bodies. Alternatively, if the planets formed in situ, the timescale for the planet to eject the planetesimals would have been shorter in systems with high-mass planets than with low-mass planets. Under both scenarios, from an evolution point of view, one would expect to find a positive correlation between low-mass planets and the presence of a remnant dust-producing planetesimal disk and, in fact, preliminary analyses of the Herschel surveys have found tentative evidence of such correlation. However, our clean sample does not confirm the presence of this correlation. Why? It could be because the true migration histories of the systems studied may be significantly more complicated than the two scenarios described above; for example, in our own solar system, it is now well established that the ice giants, Uranus and Neptune, migrated outward over a significant distance to reach their current locations, sculpting the trans-Neptunian population as they did so. Another explanation could be because the planets detected by radial velocity surveys and the dust observed at 100 μm occupy well-separated regions of space, limiting the influence of the observed closer-in planets on the dust production rate of the outer planetesimal belt. But it could also be that our sample is too small to detect such a correlation because having a clean sample that avoids the biases mentioned above comes at a price: in our sample, a positive detection of a correlation could have been detected only if the disk frequency around low-mass planet stars were to be about four times higher than the control sample.

Another aspect that we have explored is the role of planet multiplicity. Dynamical simulations of multiple-planet systems with outer planetesimal belts indicate that there might be a correlation between the presence of multiple planets and debris. This is because the presence of the former indicates a dynamically stable environment where dust producing planetesimals may have survived for extended periods of time (as opposed to single-planet systems that in the past may have experienced gravitational scattering events that resulted in the ejection of other planets and dust-producing planetesimals). However, our sample does not show evidence that debris disks are more or less common, or more or less dusty, around stars harboring multiple-planet systems compared to single-planet systems.

And how do the observed debris disks compared to our solar system? Because our sample does not show any evidence of disk evolution in Gyr timescales, we can look at the distribution of disk fractional luminosities (Ldust/Lstar; a distance-independent variable). We find that a Gaussian distribution of fractional luminosities in logarithmic scale centered on the solar system value (taken as 10-6.5) fits the data well, whereas one centered at 10 times the solar system’s debris disks can be rejected. This is of interest in the context of future prospects for terrestrial planet detection. Even though the Herschel observations presented in this study trace cold dust located at tens of AU from the star, for systems with dust at the solar system level, the dust dynamics is dominated by Poynting–Robertson drag. This force makes the dust in the outer system drift into the terrestrial-planet region. This warm dust can impede the future detection of terrestrial planets due to the contaminant exozodiacal emission. Ruling out a distribution of fractional luminosities centered at 10 times the solar system level implies that there are a large number of debris disk systems with dust levels in the KB region low enough not to become a significant source of contaminant exozodiacal emission. Comets and asteroids located closer to the star are other sources of dust that can contribute to the exozodiacal emission (and for those, Herschel observations do not provide constraints), but planetary systems with low KB dust-type of emission likely imply low-populated outer belts leading to low cometary activity. These results, therefore, indicate that there are good prospects for finding a large number of debris disk systems (i.e., systems with evidence of harboring planetesimals) with exozodiacal emission low enough to be appropriate targets for terrestrial planet searches.

Larger samples are needed to improve the statistics of the studies mentioned above, but, as we have done here, care must be taken to avoid biases. But increasing the sample size is not enough. There are two additional aspects that need to be improved upon and, with the data at hand, cannot be addressed at the moment: our ability to detect fainter debris disks (as we may only have detections for the top 20% of the dust distribution), and to detect or rule out the presence of lower-mass planets to greater distances. For the later, of critical importance is that the planet search teams make the non-detections publicly available so we can identify systems for which the presence of planets of a given mass can be excluded out to a certain distance.

Antoine de Saint-Exupéry, the world-famous writer of The Little Prince, served as an aviator for the French Aéropostale over the Paris-Dakar route, crossing the African desert for many years in the 1930s. In his book Wind, Sand and Stars he reports of having once perceived the forthcoming conflagration of a sand storm before taking off from an intermediate station in the Sahara, by observing the peculiar behavior of two dragonflies. “What filled me with joy was that I had understood a murmured monosyllable of this secret language, that I had been able to read the anger of the desert in the beating wings of a dragonfly.” This joy is not unknown to the astrophysicist. Here is a story.

Over the last year, I happened to be working on a survey of infrared emission spectra of carbon monoxide (CO) observed in young “protoplanetary” disks, the birthplaces of the plethora of exoplanets detected so far. The CO molecule is generally abundant in planet-forming regions, at disk radii comprised to within approximately 10 Astronomical Units (AU) from the central star. CO had been observed in disks for over thirty years [1], and recent instrumental developments had made possible to perform a survey of unprecedented sensitivity, spectral resolution, and sample size in the years 2007-2010 [2]. While studying the peculiar flickering behavior of CO and water emission from the disk of a variable star, I noticed that the CO spectra looked like the superposition of two emission line components, one being distinctly broader than the other [3]. I attempted a spectral decomposition analysis, encouraged by the exquisite quality of the data, and found that while many protoplanetary disks showed both CO components, some had only the narrow one [4]. By measuring the temperature (from the line flux ratios) and the disk radius of CO emission (from the line widths) in each disk of the survey, I composed the diagram shown below. When I and Klaus Pontoppidan, my collaborator and mentor, looked at it, we were astonished by the appearing of a sequence.

Figure (click to enlarge): The temperature-radius (T-R) diagram of rovibrational CO emission in disks [4]. The red and blue data points are individual disks from high quality, high spectral resolution surveys done with CRIRES at the VLT (resolving power of ~100,000) [2,5]. The sample spans a range in stellar masses of 0.5-3 solar masses (indicated by the symbol size). The location of each disk in the diagram indicates the vibrational temperature of the innermost CO gas present in the disk. At the bottom of the figure, for comparison, are shown the Solar System planets, together with the distribution of semi-major axes of observed exoplanets with Msini > 0.5 Jupiter masses [6].

Given its high dissociation temperature, CO traces the innermost disk radius where molecular gas can survive in any disk. Therefore, the location of each disk in the diagram indicates the temperature of the innermost molecular gas present in its planet-forming region. The red disks in the diagram are those found to have two CO components and are identified as “primordial”, where the inner radius is set by the stellar magnetospheric accretion or by dust sublimation (truncating the disk out to ~0.1 AU at most for the whole sample). Blue disks lack the broad CO component, and have something else going on preventing CO gas from extending all the way to the smallest distance allowed by the stellar properties…

As the CO emission analyzed here is rovibrational, the measured line ratios give a vibrational temperature, which is a sensitive thermometer of the local radiation field. The temperature-radius (T-R) diagram, taken as a whole, reveals a sequence composed of two regimes. In the inner 0.03-2 AU the temperature decreases as a power-law profile, as expected for the dust temperature in models of inner disks irradiated by the central star. This regime is identified as due to infrared pumping of CO by the local warm dust, and provides an empirical temperature profile for inner disks around solar-mass stars. The second regime takes over beyond ~2 AU, and shows an inversion in the temperature. This temperature inversion strongly points at another excitation mechanism that is known to effectively populate CO lines in low-density and cold environments: ultraviolet (UV) fluorescence [7]. In order for UV radiation to be effective at such large distances from the central star (2-20 AU), the innermost region of these disks must be largely depleted in both dust grains and molecular gas. These disks must host large inner gaps in their radial structure. Overall, CO emission suggests that all blue disks are developing or have developed large inner gaps, and some of them (filled symbols in the figure) have already been identified as “transitional” by dust emission modeling or by direct imaging. The T-R diagram has the power to provide prime targets for direct imaging campaigns, pushing inward the detection of inner gaps to radii that will become accessible to future infrared imagers (e.g. by E-ELT-METIS [8]).

But the best is yet to come. This research provides an empirical framework to investigate gap-opening processes in disks, including planet formation and migration. Comparison of the CO temperature sequence to the distribution of giant exoplanets detected so far reveals two interesting facts. The so-called “hot”-Jupiters are found at the innermost radial location of CO gas in disks, ensuring that abundant gas is present to allow gas-supported planet migration as proposed by models [9]. The distribution of exo-Jupiters, instead, rises at the break point between the two regimes in the CO diagram, supporting the existence of a link between exo-Jupiters formation and the opening of gaps in the natal disks [10], which eventually leads to their dispersion through the “debris disk” phase [11]. The journey of an exoplanet from its birth is long and can be full of surprises, ending up in the large diversity suggested by the foremost research in planetary architectures and compositions [e.g. 12]. And for us, at the horizon, stands the possibility of finding something similar to what we know here on Earth, a journey that is breathtaking for our entire world. It may still be far ahead in time, but every word we catch of this secret story of nature is welcomed with joy by those who spend their lives aspiring to hear it in full. Sometimes, these words are found in the most unexpected data, or in a diagram composed almost by chance. Sometimes, we can understand a murmured monosyllable of this secret language simply by “following the wings of a dragonfly”.

Debris disks are cold dust belts hosted by some main-sequence stars, composed of micrometer-size grains to kilometer-size planetesimals. As left-overs from planet formation, the study of these young cousins of our own solar system’s Kuiper belt can help us to better understand how planets are formed.

To do so, we need to image these disks in the visible or near-infrared, to deduce their composition and physical properties from the starlight scattered by the dust, and maybe also detect signposts of planets in their geometry. However, despite numerous surveys with the Hubble Space Telescope (HST) and the largest ground-based telescopes, only 19 debris disks had been imaged in scattered light so far. As far as planets are concerned, only 24 have been directly imaged, in significant contrast with the large numbers of discoveries by the radial velocity and the transit methods (with hundreds and thousands of planet detections respectively).

This is due to the very high contrast between the host star and the light reflected by a disk or emitted by a planet (more than a million times fainter!). To achieve such challenging detections, the instruments need to be equipped with carefully optimized coronagraphs, and with efficient adaptive optics systems for ground-based telescopes, and furthermore, the observer has to apply post-processing techniques on the resulting images to detect the dim circumstellar material.

The classical post-processing method consists of subtracting the image of a reference star from the science image to reveal material in its vicinity. However, such a subtraction is never perfect due to telescope instabilities and/or residual wavefront errors, and residual starlight still impedes the detection of cold material within 2’’ of the star (Fig. 1). New algorithms have been recently developed to solve this issue, by using large libraries of reference star images to generate a synthetic image of the star optimized to the actual science image. These new techniques improve the starlight subtraction by a factor of 10 to 100 over the classical method (Fig. 2).

Figure 1: The Principle of the classical post-processing technique: the image of a reference star is subtracted from the science image to remove the starlight. Although this method improves the contrast by a factor of 5 to 10 compared to the raw image, the telescope instabilities prevent the detection of any material within 2’’ from the star.

Figure 2: Images of the debris disk around HD181327 reduced with the classical technique (left, from [1]) and with the KLIP algorithm [2] (right, from [3]). This advanced post-processing algorithm typically improves the contrast by a factor of 10 to 100 over the classical method.

Our team has thus started the project of reprocessing the entire HST-NICMOS coronagraphic archive with such advanced algorithms to reveal new disks and planet candidates [4]. The archive is composed of images of 400 stars observed in the near-infrared between 1997 and 2008 and have been underexploited by the use of mainly old post-processing techniques. Among our recent discoveries from this project is the detection of five debris disks seen for the first time in scattered light (Fig. 3). These detections increase the total number by more than 20%. The on-going analysis and modeling of these disks should tell us more about their composition and properties and maybe present hints of possible planets.

Some planets pass in front of the stars they orbit. We call that a “transit.” One of the brightest stars with a transiting planet is HD 189733. That makes it attractive for scientific study, because the more photons one collects, the more precise a measurement can be made. The planet that orbits it is also a large one, a gas giant planet like Jupiter, similar in size and mass, but much hotter because the planet that orbits HD 189733 does so with a period of only 2 days! I am eight thousand “years” old in HD 189733b “years”!

That star also happens to have a lot of prominent star spots; from the rotational modulation of its light curve, with an amplitude of ~2% peak to valley, HD 189733 is “spottier” than 95% of all stars of similar spectral type. We know the statistics of stars in general from the Kepler mission.

Since it was first discovered to have a transiting planet approximately a decade ago, the planet HD 189733b has been the subject of scores of observational campaigns. The depth of the transit tells us the square of the ratio of the planet’s radius to the star’s radius. However, the radius of the planet’s silhouette in front of the star depends on wavelength because the grazing rays of starlight passing by the planet pass through its upper atmosphere and hence are affected differently according to the composition of the planet’s atmosphere. Hence, by taking spectra of the star-planet system during transits, we can measure the composition of the planet’s atmosphere. In this case we were searching for water, and our “divining rod” was the WFC3 instrument on Hubble. We were not sure we’d detect it – prior attempts had failed: with the older NICMOS instrument the results were plagued by systematic effects, and even two recent attempts with the new WFC3 instrument had failed. The first attempt failed because the bright star saturated the detector, and second attempt failed because of a miscommunication about a software update that caused the planet to be observed when the Earth itself was between Hubble and the star – blocking out the scene at the critical hour when the planet was transiting the star. We hoped that this third attempt “would be the charm”!

It was. We obtained excellent WFC3 data on June 5, of 2013 and found a clear signature of water vapor at 1.4 microns in the spectrum taken during transit that was not there before or after the transit. In a recent press release, dated July 24, 2014, my colleague N. Madhusudhan describes this measurement, and two others, as the first measurements of a chemical compound in exoplanets. To be sure, although we and others have detected water vapor before in exoplanets, those detections were tentative enough that the water concentration could not be quantified much better than to say that it was greater than zero.

For planet HD 189733b, there were some that expected we would not see the signature of water vapor. While they expected that gaseous water ought to be present, they imagined that it would be obscured from our view by a haze layer, much like the buildings of Los Angeles or Beijing can be obscured by smog. This expectation was based on prior Hubble observations made with the instruments STIS and ACS.

The working hypothesis, developed over the past few years by Frederic Pont and his colleagues, to explain the ensemble of data from the Hubble and Spitzer space telescopes has been that a haze layer in the upper atmosphere of the planet Rayleigh-scatters light, creating a circular silhouette of the planet that is largest in the ultraviolet and smoothly decreases ever so slightly with wavelength into the near-infrared until it bottoms out and is nearly flat in the thermal infrared. Figure 1 comes from our ApJ paper (McCullough et al. 2014, 791, 55) . One way to shoe-horn the new detection of water vapor into what had become the consensus model is to presume that the water vapor occurs at higher altitudes than the haze layer. Although that is still plausible, in investigating the various models I became less and less confident that the consensus was correct. Another model may apply equally well or better.

Figure 1. Transmission spectrum of the exoplanet HD 189733b. Observations: The upper WFC3 spectrum from our analysis (black open circles) is as-observed; the mean transit depth for the WFC3 data is approximately at the same level as the ∼1 μm end of the ACS data (blue open circles) reported by Pont et al. (2008), which had been corrected for an assumed unocculted star spot level of 1%. The lower WFC3 spectrum (black filled circles) has been shifted down by 300 ppm to better match the ACS data (blue filled circles) corrected for an unocculted star spot level of 1.7% by Pont et al. (2013). Models: One model (upper, red line) combines two effects: 1) unocculted star spots with temperature T(spot) = 3700 K and spot fractional area δ = 0.056, and 2) a clear planetary atmosphere of solar composition, a mixing ratio for water of 5×10-4, and zero alkali metal lines (Na and K) for a gas giant planet with physical parameters commensurate with HD 189733b. The other model (lower, orange line) is solely the contribution of the unocculted star spots of the first model. Both models have been smoothed with a Gaussian of FWHM=0.089 μm for clarity.

The core of my idea was expressed also by Pont and his colleagues before our WFC3 observations. They had noted that uncertainty about spots on the face of the star could create uncertainty in the slope in the spectrum from the UV through the visible to the near-infrared, nominally attributed to Rayleigh-scattering in the planet’s atmosphere. Recall that the measurement, namely the transit depth, depends on the square of the ratio of the planet’s radius to the radius of the star. Obviously, either the planet or the star can affect the ratio. If the star has spots off the transit chord, i.e. they are on the star but never occulted by the planet, those spots can mimic the “Rayleigh-scattering slope.” Here’s why. The spots are cooler and redder than the stellar photosphere. Such a spot will cause the transit depth to be deeper than it would be otherwise, which we might misinterpret as the planet’s radius being larger. And because cooler spots are also redder than the photosphere, the spot is relatively darker in the blue than the red. Thus, we might interpret the observed fact that the transit is deeper in the blue than the red, as either (A) the planet being larger in the blue (i.e. Rayleigh scattering) or (B) the star having unocculted spots that have not been fully accounted for. Pont’s team was aware of all that, but their best estimate of the number and temperature of spots seemed too small to them to account for all of the spectrum’s slope, which they therefore attributed to model A. When we examined the same data, and added the new WFC3 data showing a water-vapor feature in the near infrared, we re-interpreted the combination of old and new data with model B.

The overall lesson from this story is that even for one of the best objects to study, namely one of the closest transiting exoplanets to Earth, and a large gas-giant planet at that, the interpretation of the observations is still unsettled. That is unsatisfying, challenging, and fun, all at the same time. So it is with science. Hopefully a more thorough interpretation of existing data and/or even better data obtained with future instruments or telescopes such as JWST will solve this puzzle.

It’s exciting to witness history being made, especially if it’s a long-awaited event. The afternoon of October 6, 1995 was that time. Several hundred astronomers were in Florence, Italy for the ninth in a series of scientific meetings on Cool Stars, Stellar Systems, and the Sun. The Cool Stars series was started by Andrea Dupree in 1980 in Cambridge, Massachusetts, and they continue to be held every two years. Cool Stars 9 was the first to be held outside the U.S., and Roberto Pallavicini from Florence’s Arcetri Observatory led the group that organized and hosted the meeting.

Florence is a delightful place to be under almost any circumstances: warm, sunny, good food, and great sights. But everyone was very focused that afternoon. In the morning we were told that there’d be a special short talk later, and the buzz over lunch was that Michel Mayor of the Geneva Observatory, working with Didier Queloz, had a rock-solid detection of a planet around another Sun-like star to report.

When we returned to the meeting hall after lunch I made sure to get a seat up front. Nobody really knew anything yet, just that a big announcement was coming. We all knew Mayor and Queloz were searching for planets, so what else could it be? But which star, and what kind of planet? All the brighter Sun-like stars were well known to many of us, so it was probably a star we’d heard of and studied ourselves.

We then had to sit through most of the afternoon’s scheduled talks; they were selected to be interesting in the first place, but at this point were being eclipsed by the Main Event. As the time grew near for Mayor’s presentation some people with television cameras started filtering into the back of the room. Mayor showed us his observations and conclusions, and there was little to argue with. There was warm and enthusiastic applause: the prey so long sought had been nabbed.

So why did it take so long? It’s been pointed out that 51 Pegasi, the host star to this planet, was misclassified as being evolved. But that in itself wouldn’t preclude detecting the planet that was found. A big part of the reason is simply that 51 Peg B – the planet – was so very different than anyone ever expected. Or, to be more precise, it was located where we did not think we’d find planets: close in to the host star.

The conventional wisdom was that other planetary systems would be like ours, with analogs to Jupiter at great distances and with long periods that would take years of careful measurements to detect. At the time I imagined the process Mayor and Queloz must have gone through. They’d been observing 51 Peg and stars like it for some time, hoping to see very small changes in the motions of the stars that would indicate a planetary companion. 51 Peg showed larger variations than they could make sense of, so they started observing it more often, maybe every month instead of a few times per year. And still, 51 Peg showed some kind of motion going on but with no clear period. So they started observing it every week, and still no period to the variations. It was only when they measured it every night for a few weeks that the 4.2-day period became blindingly obvious. They had it!

Once the realization dawned that these “hot Jupiters” could exist so close to a star they became fairly easy to find because they produce much larger signals than more distant planets. The race was on and over the next several years there were dozens and dozens of new exoplanets announced by Mayor’s group and a competing group led by Geoff Marcy in California. There was no turning back. History was made.

Scientific meetings like Cool Stars 9 often feature an invited talk at the end by a senior researcher who provides a summary of the high points and his or her views on their significance and implications for the near future. That person that time was Jeff Linsky, from Boulder CO. But Linsky played hooky to visit museums the afternoon that Mayor made his presentation and so he missed it. His conference summary mentions nothing of the one talk that will be remembered longest.

Finally, it is now nearly 20 years since Cool Stars 9. I was conscious of events at the time, but the details are getting fuzzier, and the recollections of others may sound different.

This Month’s Featured Author

Dr. Brian Williams received his B.S. from Florida State University in 2004 and his Ph.D. from North Carolina State University in 2010. He was a NASA Postdoctoral Fellow at NASA Goddard Space Flight Center for three years, after which he worked as a research scientist at NASA GSFC with Universities Space Research Association. He arrived at STScI in February of 2017, and is currently a Support Scientist in the Science Mission Office. His research interests include supernovae and supernova remnants, shock physics and particle acceleration, and dust in the interstellar medium.